NEW YORK (AFP) - US Secretary of State John Kerry insisted Tuesday (Sept 20) that hopes for a ceasefire in Syria remain alive after meeting with Russia and key powers with a stake in the civil war.

In brief remarks to reporters as he left a New York hotel after a meeting of the International Syria Support Group, Kerry said talks would reconvene later this week.

"The ceasefire is not dead," Kerry insisted, one day after the Syrian military declared a week-old truce over and launched new bombardments on rebel held cities.

The United Nations envoy Staffan de Mistura confirmed that there was still hope of reviving the ceasefire, but admitted that delegates agreed it was in danger.

The 23-nation ISSG, chaired by Kerry and his Russian counterpart Foreign Minister Sergei Lavrov, met in New York on the sidelines of the UN General Assembly.

The talks were brief and, participants said, tense.

"The mood is that nobody wants to give this thing up," Britain's Foreign Secretary Boris Johnson told reporters.

"Quite frankly the Kerry-Lavrov process is the only show in town and we've got to get that show back on the road."

His French counterpart Jean-Marc Ayrault agreed that the meeting had been tense but argued other countries should now help Moscow and Washington overcome their differences.

"It was a fairly dramatic meeting, the mood was gloomy. Is there hope? I can't answer that yet, but we should do everything we can," he said.

"The US-Russian negotiation has reached its limit. There's a lot left unsaid. The Russians and the Americans can't do it alone."

The ministers are in New York for the week to attend the UN General Assembly and officials said they would try to get together again to talk about Syria.